1. Field of the Invention
The present invention is directed toward a cement mixer truck, and more particularly toward a system and method for measuring the weight of the contents of a cement mixer truck.
2. Description of the Related Art
FIG. 1 is a schematic of a cement mixer truck 100 according to known art. The cement mixer truck 100 has a mixing chamber 110 (also referred to as a tub) that carries a load of cement or possibly other types of material with fluid-like qualities. Typically, a front mount 120 and a rear mount 130 rotatably support the mixing chamber 110 and its load. The front mount 120 may include rollers, gears, bearings and other structural units and/or drive mechanisms to support a portion of the weight of the mixing chamber 110 and load, and drive rotation of the mixing chamber 110. Additionally, the rear mount 130 may include rollers, gears, bearings and other structural and/or rotatable devices to support a remainder of the weight of the mixing chamber 110 and load and facilitate rotation of the mixing chamber 110.
The cement mixer truck 100 includes one weight measurement device (not shown). Typically, the front mount 120 includes the weight measurement device (i.e., scale). Although the weight measurement device can be calibrated to give a fairly accurate weight measurement when the mixing chamber 110 carries a predefined volume of material of a predefined density, the weight measurement becomes inaccurate as load material is either removed from or added to the mixing chamber 110. In fact, if for example the weight measurement device is calibrated to give a fairly accurate weight measurement when the mixing chamber 110 carries a full load, the weight measurement becomes increasingly more inaccurate as more and more load material is removed from the mixing chamber 110. In addition, if the mixing chamber 110 carries a load material of a density different from the predefined density used to calibrate the weight measurement device, the weight measurements generated by the weight measurement device will be inaccurate for at least one or more ranges of load volume of the mixing chamber 110. The errors will depend upon the extent that the density of the load material differs from the predefined load material density used to calibrate the weight measurement device.
The weight measurement inaccuracies exist because the cement mixer truck 100 has only one weight measurement device (typically installed as a part of the front mount 120) for making a force measurement, although the mixing chamber 110 has two or more positions of support (i.e., front and rear mounts 120 and 130) over which the weight is distributed. A weight measurement system having only one weight measurement device to measure the weight of the mixing chamber 110 and load supported at two or more positions yields an accurate weight measurement only if a position for the center of mass of the mixing chamber 110 and load is known. However, although the center of mass of the mixing chamber 110 and load may be known for a predefined load, the center of mass shifts as load material is either removed from or added to the predefined load. Thus, since the center of mass is unknown, the weight measurement device of the known art cannot produce an accurate weight measurement. In order for the known art to produce an accurate weight measurement, the center of mass must be determined, which requires knowledge of the weight of the load, which is unknown.
FIG. 2 is a schematic of the mixing chamber 110 of FIG. 1 being acted upon by external forces. As illustrated, forces F1 and F2 applied by the front mount 120 and the rear mount 130, respectively, support the mixing chamber 110. However, since only one weight measurement device (not shown) is installed as part of the front mount 120, only F1 will be measured. F2 is not measured, and thus remains an unknown quantity. A reference coordinate system 202 is shown having an origin O. The front mount 120 is located at distance x1 from the origin O, and the rear mount 130 is located a distance x2 from the origin O. When the mixing chamber 110 contains a full load, the mixing chamber 110 and load is known to have a mass M, and the center of mass of the mixing chamber 110 and load is known to be located a distance xcm from the origin O. A gravitational force M·g acts upon the mixing chamber 110 and load at the center of mass xcm. Since the mixing chamber 110 and load are static (i.e. at rest), the sum of all translational and rotational forces acting on the mixing chamber 110 and load must be zero. Equation (1) expresses that the net translational force acting on the mixing chamber 110 and load is zero, and equation (2) expresses that the net torque acting on the mixing chamber 110 and load with respect to the origin O is zero.F1+F2=M·g  (1)F1·x1+F2·x2=M·g·xcm  (2)
Now assume that a portion of the load has been removed, and an operator wishes to discover the remaining weight M·g of the mixing chamber 110 and load. As illustrated, F′1 and F′2 are the new forces applied to the mixing chamber 110 by the front mount 120 and rear mount 130, respectively, and the center of mass has shifted to x′cm measured with respect to the origin O. One replaces F1, F2, M and xcm of equations (1) and (2) with F′1, F′2, M′ and x′cm, and simultaneously solves equations (1) and (2) for M′ (eliminating the unknown force F′2), obtainingM′=[F′1/g]·[(x2−x1)/(x2−x′cm)]  (3)
Equation (3) generates an accurate value for M′ as long as the center of mass location x′cm is known. However, x′cm is not known. The method as practiced by the known art substitutes xcm for x′cm in equation (3) to obtain an estimate for M′. Thus, the mass M′ computed by equation (3) will be larger than the true mass of the mixing chamber 110 and new load. (The mass M′ computed by equation (3) may also be smaller than the true mass, depending upon the value of x′cm). By using only one force measurement taken at one support position of the mixing chamber 110 (when the mixing chamber 110 is supported at two or more support positions), an inaccurate value for the mass, and hence weight, of the mixing chamber 110 and load is generated.
What is needed is a system and method for accurately determining the mass and weight of a cement mixer at all load volumes.